Soutenance de thèse / PhD Defense - Ibrahim Buba Garba

Ibrahim Buba Garba, doctorant dans l'équipe Théorie quantique des matériaux soutient sa thèse le mardi 5 septembre 2023 à 14 h.

IMPMC - Sorbonne Université - 4 place Jussieu - 75005 Paris - Barre 22-23, 4e étage, salle 401
 

Ab initio Characterization of Anharmonic Materials

Abstract

Accurate prediction of thermodynamic and transport properties of materials from first principles requires an understanding of anharmonicity resulting from phonon-phonon interaction. The standard quasiharmonic approximation neglect explicit temperature dependence of phonons and is invalid at high temperature, especially near a phase transition, or even at low temperatures if the energy profile is shallow or if the quantum nature of nuclei cannot be disregarded, which warrants the need for anharmonic methods. Furthermore, the perturbative treatment of anharmonicity can be problematic in the presence of imaginary phonons, with no suitable ground state upon which a perturbative expansion can be built.We present a method of computing anharmonic phonons, in which the only assumption appears to be the form of the dynamical matrices. Starting from a given crystal geometry and a grid of q-points in the irreducible Brillouin zone, we define a trial dynamical matrix using an orthonormal symmetrized basis whose values can be randomly generated. The trial dynamical matrices are optimized by fitting to forces sampled in standard ab-initio molecular dynamics and with Langevin dynamics, based on the temperature-dependent effective potential method. By way of example, we apply it to study anharmonic phonon renormalization in weakly and strongly anharmonic materials, reproducing the temperature effect on phonon frequencies, stabilization of high-temperature phases in metals, and phase transition in ferroelectrics, in good agreement with the experiment.

Jury

• M. Marco Pala, C2N, Univ. Paris-Saclay, Rapporteur 
• M. Nicolas Horny, ITheMM, Univ. de Reims Champagne-Ardenne, Rapporteur
• Mme. Magali Benoit, CEMES – CNRS, Toulouse, Examinatrice
• Mme. Nedjma Bendiab, Néel Institute, Univ. Grenoble Alpes, Examinatrice
• M. Fabio Finocchi, Institut des NanoSciences de Paris, Examinateur SU 
• M. François Bottin, CEA/DAM Ile-de-France, Invité
• Mme. Giorgia Fugallo, LTeN, CNRS Nantes, Invitée
• M. Lorenzo Paulatto, IMPMC CNRS, Directeur de thèse